Magnetic field sensors can be used to sense an angle of rotation of a shaft or other object. For example, a magnet can be mounted on the shaft, and a magnetic field sensor can be arranged proximate the magnet in order to sense a magnetic field induced by the magnet as it rotates with the shaft. When the magnetic field sensor is mounted next to or adjacent the shaft, i.e., off of the axis of rotation of the shaft, the sensor is often referred to as an “off-axis” magnetic field angle sensor. Off-axis magnetic field angle sensors often are implemented when the end of the shaft is unavailable as a location for the sensor or there simply is not space available on the shaft. Generally, an “on-axis” magnetic field sensor, then, is one in which the sensor is mounted at or near the end of the shaft, generally in-line with or on the axis of rotation. In some embodiments, on-axis magnetic field angle sensors can comprise gradiometers, such that an on-axis magnetic field gradient sensor measures a gradient of the magnetic field on the rotation axis. The gradient can be measured in sufficient approximation in some applications by sampling the magnetic field at samplings points close to and on opposing sides of the axis and combining the samples, such as by subtraction.
In many applications there is a general preference for magnetic field angle sensors, whether off- or on-axis, to be inexpensive while also being robust with respect to external magnetic fields and other disturbances and insensitive to assembly tolerances. One particular application for magnetic field angle sensors is in brushless DC (BLDC) motors for the detection of the shaft position in rotation. BLDC motors present a challenging environment for magnetic field sensors as they typically include strong rotating magnets and copper windings carrying large currents, both of which produce time-varying magnetic fields. These fields are strongly inhomogeneous, which makes it difficult to cancel their effects from an angle sensor signal, and their strength can be hard to predict. Overall sensor size is also a consideration in order to avoid adding additional mass or volume to the BLDC motor. Smaller gradiometric-type sensors also can be more efficient in canceling background magnetic field disturbances. In general, therefore, there are numerous drawbacks associated with conventional magnetic field angle sensors.